Shira uses 6 small skeins of yarn for each scarf she knits. In order to be ready for the next holiday, she needs to make 15 scarves. How many small skeins of yarn does she need?
To make 1 scarf, Shira needs 6 skeins. To make 2 scarves, she needs $6+6=12$ skeins. To make 3 scarves, she needs $6+6+6=18$ skeins. Observe that the number of skeins she needs is just 6 times the number of scarves she wants to make. Therefore, she needs $15\times6=\boxed{90}$ skeins to make 15 scarves.